There has long been a need for methods of cleaning and inspecting tubulars, particularly offshore riser pipe, on site. Typical methods used today include disassembling the riser pipe from a rig, transporting the riser pipe to a yard, and there conducting inspection and testing of the tubular using well known techniques. Such a method is not only expensive and time consuming, but also very disruptive of normal operations on the rig.
Thus, there remain a need for a system and method of inspecting tubulars on site to minimize down time of the rig, and to save the costs of transporting and returning the tubulars under inspection.
Even the techniques used at the yard for the testing and inspection of tubulars have certain drawbacks. Various techniques have been developed to detect flaws in structures, particularly welds in such structures. The ability to detect flaws in structures such as tubulars in drilling and production rigs and pipelines is especially critical before any catastrophic failure occurs.
Ultrasonic testing of metal structures has proved to an effective and practical tool for nondestructive testing (NDT). Known ultrasonic techniques typically yield reliable examination results. However, some geometries make known ultrasonic techniques difficult or even impossible to apply, or yield inaccurate results.
One technique that has gained common acceptance in the NDT field is referred to as the echodynamic technique. This technique consists of measuring the duration of the defect echo in axial or circumferential tube direction when the ultrasonic probe (in pulse-echo mode) is moved over the defect. Such a defect may involve slag, porosity, stress cracking, or other anomalies from the anticipated metal grain structure. In the pulse-echo mode, the depth of a defect is calculated from the probe displacement distance at which a defect echo was picked up. To detect the defect, the amplitude of the defect echo should be above noise level. However, many defects that are of particular concern escape detection if they are oriented in a particular way relative to the applied pulse echo, because this technique relies on the reflectivity of the defect. In fact, the pulse echo technique is used in the present invention for corrosion mapping in determining pipe wall thickness. However, as previously described, the pulse echo technique may miss certain flaws, and this fact has lead to the development of other testing techniques.
The Time of Flight Diffraction technique (TOFD) was developed by the AEA's Harwell Laboratory in Britain in the mid seventies as a method of accurately sizing and monitoring the through-wall height of in-service flaws in the nuclear industry. For weld inspection, it was quickly recognized that the method was equally effective for the detection of flaws, irrespective of type or orientation of the flaw, since TOFD does not rely on the reflectivity of the flaw. Rather, TOM detects the diffracted sound initiating from the tips of the flaw.
In TOFD, a transmitting probe emits a short burst of sound energy into a material and the sound energy spreads out and propagates in an angular beam. Some of the energy is reflected from the flaw but some of the energy is incident to the flaw and is diffracted away from the flaw. A fraction of this diffracted sound travels toward a receiving probe. The diffracted signals which are received by the receiving probe are time resolved using simple geometry calculations and are graphically displayed in a grey scale form.
While the TOFD technique has proved effective for many geometries, there remains a need for a method and system for detection of flaws from within a cylindrical structure, such as a pipe or riser stanchion. The present invention is believed to be the first structure and method of NDT using TOFD from within a tubular such as a riser pipe.